Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

Balbir Singh
2010-12-13T12:39:44Z

* Avi Kivity <avi@redhat.com> [2010-12-13 13:57:37]:

> On 12/11/2010 03:57 PM, Balbir Singh wrote:
> >* Avi Kivity<avi@redhat.com> [2010-12-11 09:31:24]:
> >
> >> On 12/10/2010 07:03 AM, Balbir Singh wrote:
> >> >>
> >> >> Scheduler people, please flame me with anything I may have done
> >> >> wrong, so I can do it right for a next version :)
> >> >>
> >> >
> >> >This is a good problem statement, there are other things to consider
> >> >as well
> >> >
> >> >1. If a hard limit feature is enabled underneath, donating the
> >> >timeslice would probably not make too much sense in that case
> >>
> >> What's the alternative?
> >>
> >> Consider a two vcpu guest with a 50% hard cap. Suppose the workload
> >> involves ping-ponging within the guest. If the scheduler decides to
> >> schedule the vcpus without any overlap, then the throughput will be
> >> dictated by the time slice. If we allow donation, throughput is
> >> limited by context switch latency.
> >>
> >
> >If the vpcu holding the lock runs more and capped, the timeslice
> >transfer is a heuristic that will not help.
>
> Why not? as long as we shift the cap as well.
>

Shifting the cap would break it, no? Anyway, that is something for us
to keep track of as we add additional heuristics, not a show stopper.

--
Three Cheers,
Balbir